1. Field of the Invention
The present invention relates to image signal processing, and more particularly, to a method and circuit for correcting contour components of a luminance signal.
The present application is based on Korea Patent Application No. 96-59842 which is incorporated herein by reference for all purposes.
2. Description of the Related Arts
Spatial frequency is a measure of how rapidly a parameter changes with respect to position. When the spatial frequency is determined in a prescribed spatial direction, it is analogous to temporal frequency, which is a measure of how rapidly a parameter changes with respect to time. In television systems using horizontal scanning lines, horizontal space can be conformably mapped into time by the scanning process, so that horizontal spatial frequency of the image intensity is mapped into temporal frequency in the video signal describing the image.
In video cameras using a single pickup device, a color pattern filter can be used to filter light reaching the pickup device so that a color signal can be extracted from the electrical signal output by the pickup device. The color pattern filter usually includes stripes capable of transmitting light of three different colors to the pickup device, which can be a vidicon or may be a solid-state imager such as a line-transfer charge-coupled device. The direction of the stripes is perpendicular to the direction of scanning line in the camera, which usually is in a horizontal direction. The stripes of the same color have uniform width, but the stripes of different colors preferably have different widths in order to simplify the separation of color components from the output signal of the pickup device. The respective width associated with each color is usually scaled according to the contribution of the color to a total luminance signal. If the color filter, for example, includes red-transmissive, green transmissive and blue-transmissive stripes, the green-transmissive stripes (which contribute the most to the luminance of the signal) will be the widest and the blue-transmissive stripes (contributing the least) will be the narrowest. The signals picked up by the narrower width stripes have poorer signal-to-noise ratio (S/N), particularly in the higher horizontal spatial frequencies containing contour information. When a video camera is used with a video transmission system, the color signals are converted to wideband luminance and narrow-band color-difference signals. The poorer S/N ratios of the individual colors which contribute less to the luminance are not of much concern since contour enhancement or video peaking is usually carried out on shared luminance high frequencies rather than on individual color signals.
However, video equipment exists in which the color signals are not combined to form luminance and color-difference signals, e.g., a RGB-type digital video transmission system in which the red (R), green (G) and blue (B) signals are separately digitized and coded. In such equipment, contour enhancement or video peaking is commonly carried out for the red (R), green (G) and blue (B) color signals themselves. If a color contour that has poorer S/N is corrected with another color contour that has better S/N, an image that has less apparent noise can be obtained. A random noise in the green (G) color signal is not correlated with random noises in the red (R) or blue (B) color signals. Thus, if a random noise component in the G signal and a random noise component in another color signal is added by orthogonal vectors rather than inphase vectors, the S/N characteristics in the high-frequency band are enhanced during contour correction of the other color signal.
In a conventional method for enhancing the contour components of the luminance signal, contour components in a horizontal and vertical directions are detected and added, and the added result is combined with the original signal.
FIG. 1 is a block diagram of a conventional circuit for enhancing contour components of a luminance signal. FIG. 2 shows the input-out characteristic of a non-linear processor in the circuit of FIG. 1.
In the circuit of FIG. 1, the contour detecting filter 11 receives an original luminance signal and performs a band-pass filtering operation to detect horizontal contour components. The characteristics of the contour detecting filter 11 may be designed according to the specification of a processing system. Usually, a traversal filter having coefficients symmetrical against a center coefficient is used. One example of a simple contour detecting filter 11 that can be used is a 3-tap band-pass filter whose coefficients are -0.5, 1 and 0.5.
Higher levels in the output signals of the contour detecting filter 11 may be regarded as the horizontal contour components while lower levels may be regarded as noise, etc. If the lower level components are enhanced, the noise component is emphasized and the picture quality deteriorates. The coring processor 13a eliminates low-level contour components among the horizontal contour components by outputting zero-level when a signal level is lower than a coring value and outputting the signal level subtracted by the coring value when the signal level is higher than the coring value. This coring value is an externally-preset parameter.
In the prior art, the coring value is typically set to a fixed value by the manufacturer. Accordingly, if there is much noise in the signal, the performance of the contour adjustment deteriorates because the noise component which is larger than the coring value is regarded as the contour component. On the other hand, if there is little noise in the signal, the detection of the contour component is ineffective because the contour component which is smaller than the coring value is ignored.
The gain processor 13b multiplies the output of the coring processor 13a by a gain, which is another externally-preset parameter, to control the gain of the signal output by the coring processor 13a. The limiter 13c limits the gain-controlled contour components within a predetermined threshold value by clipping the gain-controlled contour components according to the threshold value or a limiting value which is another externally-preset parameter.
As described above, the coring processor 13a, gain processor 13b and limiter 13c are referred to collectively as a nonlinear processor 13. This processor 13 carries out non-linear processing of the contour components according to the preset coring, gain and clipping threshold values.
Meanwhile, the vertical contour components, which are detected only in a system employing a line memory (not shown), are detected by calculating the difference between a current pixel and a corresponding pixel in an adjacent line. The vertical contour components are added to the horizontal contour component to yield a contour component signal of the pixel. In general, however, signal processing is carried out only for the horizontal contour components and not vertical contour components in order to reduce the hardware required, i.e., a line memory, etc., in a simplified image processing system.
The contour components signal detected by the contour detecting filter 11 and processed by the non-linear processor 13 are added to the original luminance signal in an adder 15 to become a contour-enhanced luminance signal which is known to improve the picture quality.
According to the aforementioned conventional technology, an image signal with improved picture quality can be obtained by enhancing the contour components of a luminance signal. Here, the types of contour detecting filters used can be varied by known means depending on the desired characteristics of the system. Also, the gain processor and limiter of the non-linear processor may be modified by adjusting the gain and the threshold value.
However, if the externally supplied coring value of the coring processor is a fixed value, the contour enhancement effect is reduced depending on image characteristics. In other words, if a small coring value is set, the noise components of the image are also contour-enhanced, which emphasizes the noise. On the other hand, if a large coring value is set, the value obtained by subtracting the coring value from the actual contour components can suppress the actual contour components as well as.
Accordingly, the need remains for a system and method for adaptively enhancing contour components according to image characteristics by performing a coring operation with respect to a contour component by use of a coring value which is adjusted such that the value depends on the noise figure of the input luminance signal.